Use self.assertIsInstance()

Replace "assert type(...) is Foo" with "self.assertIsInstance(..., Foo)"
this produces a more meaningful test error message and fixes pep8 rule
E721[0]

[0] https://www.flake8rules.com/rules/E721.html

(cherry picked from commit f9a4c92)

zaza/openstack/charm_tests/ceilometer/tests.py

@@ -126,7 +126,7 @@ def test_900_restart_on_config_change(self):
         current_value = openstack_utils.get_application_config_option(
             self.application_name, config_name
         )
-        assert type(current_value) == bool
+        self.assertIsInstance(current_value, bool)
         new_value = not current_value
 
         # Convert bool to str